Continuity mistake: When the treehouse is about to explode, we see Bertram say he's getting out of there and go down the ladder. Then after some scenes we see him running out of the tree house when he was supposedly already down on the ground.
Continuity mistake: Scotty Smalls has a black eye the second day he goes to the sandlot. After Benny teaches him how to catch, he turns around and looks at the fence, and his black eye is healed. When when we see him going home his black eye reappears.
Continuity mistake: It is established that "The Sandlot" is a relatively vacant lot, mostly dirt, with grass mostly in the infield areas. But numerous times in the scenes involving trying to retrieve the baseball, the background shows a manicured yard with a yellow house just off to the side. It's not in the distance, it's right next to the treehouse. And it doesn't appear that this is just another angle, as we see the treehouse from a distance numerous times, and there is no grass or house anywhere in its area, let alone a full back yard with a grill.
Factual error: When Franklin grabs the ball and hurdles the fence, in the next shot we see the dog (a tiny bulldog) also jump the fence and chase him. No dog that small would reasonably or realistically jump a 12 ft fence, even with a very long-running start. It would be physically impossible. This was purely for comical purposes, nothing else.